The method of electro-photographic printing is well known in the art. In this method, a photoconductive surface, typically on a drum, is charged to a uniform potential. The charged photoreceptor and/or photoconductive surface are exposed to a light image from, for example, a writing head laser that discharges specific areas on the photoconductive surface. This records an electrostatic latent image on the photoconductive surface. After the photoconductive image is recorded, the latent image is developed. The developed image is then transferred to an intermediate transfer member such as a blanket and subsequently transferred to a substrate, such as paper.
Often, printing the same image repeatedly at the same position causes ghost appearances on subsequent prints due to memory of the previous image. Memory of the previous image may cause variations in the change in potential obtained from exposure to the light image. Such fluctuation may lead to a fluctuation in the development of the latent image. Ghost may occur due to change of surface properties of blanket or photoconductor, like surface energy or roughness, by foreign coating in the image area or background; by deterioration of mechanical properties, like resilience, etc. Typically, developing differences occur between ex-image areas and ex-nonimage areas of the photoconductive surface and/or blanket leading to undesired ghost appearances.
The appearances of ghosts may prompt early replacement of the photoconductive surface and/or the intermediate transfer drum or blanket and thereby increase the print cost.
A known solution that may be implemented for some electro-photography printing devices is use of a seamless drum with a perimeter which is not equal to the print length. In this model, every subsequent image may be shifted on the drum relatively to previous images so that repeated development in the same area may be avoided. This solution may be difficult to implement in large electro-photographic printing, e.g. liquid electro-photographic printing devices that typically use drums with seams or drums on which photoreceptor sheets are mounted.
A similar solution for overcoming ghosts on intermediate transfer members is described in PCT Publication No. WO2007018500 to Hewlett Packard Development Co., entitled “Apparatus And Method For Life Enhancement Of A Print Blanket In Electrostatic Printing”, the disclosure of which is incorporated herein by reference. PCT Publication No. WO2007018500 describes an apparatus and method for reducing degradation of a print blanket used in electro-photographic printing by changing an image location and/or orientation during the printing process on the print blanket.
U.S. Patent Application Publication No. 20020044189 to Kenichiro Kitajima et al, entitled “Color Image Forming Apparatus” describes an image forming apparatus including a look-up table for gradational correction of exposure amount to correct for ghost images. When switching from a normal mode to exposure amount reduction mode, the exposure amount is reduced according to the pre-saved data on the look-up table. The exposure amount is reduced on a global basis and is not specific to ex-image and ex-nonimage areas on the latent image.